$50 for a vet fund would definately not be enough. I just spent $150 on a vet today and I spent $180 a few months ago and those were not serious health issues. The last thing you want is to get them, they get sick in a year and you are unable to pay for their vet care. My vet only charges $25 for a visit. The expensive part is the meds and the services like xray, ultrasounds, painkillers, antibiotics and such. 5 days of antibiotics for one of my guinea pigs is almost $50.
Glad you are building a cage. They are great and easy to reconfigure if you want to add space, change it etc...
Fleece is fairly economical. It needs to be swept/vacummed 1-2 times a day for poops. Then shaken, washed and dried 1-2 times a week. You'll want to be sure to prep the fleece properly so it wicks away the pee. You can get all kinds of styles, colors, patterns. I would say for optimal poop camoflague get a darkish patterned fleece. (Unless of course your piggies are really white/light haired then get a mid tone color) If you have to buy everything new you will probably spend up to $15-20 for 3 yards of fleece for 2 changes if you don't drape the fleece over the edges. you'll need 3 towels at least per changing so $18 approx for towels. Many of my fleece and towels last 3-4 years before they get pitched/replaced.
Coroplast can cost around $10-20 for a 4x8 foot sheet that will be just enough for a 2x5. Not sure how high you will want to make your cage but the higher you go the more grids and connectors you will need. If you have a dog, no matter how docile it maybe you will want to dog proof the cage. Those shelving grids work great for easy to use lids. The store should be able to cut them to the size you want. You'll want one 12 inch wide and one 16 inch wide and put the bigger one in front for easy access and cleaning. You'll also want to pick up a bunch of zip ties (zip cables) to add extra security to your cage and stand. So depending on those prices/sales and what not could be about $80 for grids and lid.
Kleenmamas (online only) has THE best hay and pellets ever for guinea pigs. You will have to order in bulk but that makes things MUCH cheaper than pet stores and then you are not supporting them, your supporting someone good! So about $40 for hay (depending where you are) and $30 for pellets for a 6 month supply.
Veggies vary depending on where you are, where they come from, what time of year etc...
https://www.guinealynx.info/forums/viewtopic.php?t=60864&highlight=project
Here is an in progress analysis based on several people over several months on what they spend for care of their guinea pigs...
Don't forget about medical supplies you should/will need on hand. Qtips, cotton balls, mineral oil (cleaning ears, penis, anal sacks), coconut oil (for cleaning grease gland), nail clippers, infant gas drops, acidophilous pills for diareah, a variety of syringes for hand feeding, dosing meds, cleaning and so much more...
Then theres toys, activities. There are so many cute fun things you can do to keep them entertained. My friend has a great collection of the fun things she has done with her cage over the years...
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Well I think I blabbed your eyes out.... Hehe can't wait to see your new babies...